The present invention relates to electrode structures for use in, for example, bipolar electrolysers, particularly a modular bipolar electrolyser, and also in fuel cells.
Monopolar filter-press type membrane electrolysers are well known, for example as described in our GB 1,595,183. Such electrolysers are readily made, inexpensive and easily assembled.
Bipolar electrolysers are known in the art, for example as described in our GB 1,581,348.
In bipolar electrolysers of conventional design for use in the electrolysis of aqueous solutions of alkali metal chloride the bipolar unit comprises an anode which is suitably in the form of a plate of a film-forming metal, usually titanium, the plate carrying an electrocatalytically active coating, for example a platinum group metal oxide, and a cathode which is suitably in the form of a perforated, eg foraminous plate of metal, usually nickel or mild steel, the anode and cathode being electrically conductively bonded to each other to form a bipolar unit. Separators are positioned between successive bipolar units arranged in series in the electrolyser so that the anode of one bipolar unit faces the cathode of the adjacent bipolar unit with a separator disposed therebetween. The electrolyser also comprises terminal anode and cathode units.
In operating an electrolytic cell of the bipolar type, it is advantageous to operate with as small a distance as possible between the anode and cathode (the anode/cathode gap) in order to keep ohmic losses, and hence the cell voltage to a minimum.
The separators are generally adjacent or even in contact with the cathode and in order to achieve a small anode-cathode gap without at the same time damaging the separator it is necessary to exercise considerable care in order to manufacture electrodes having a suitable degree of flatness and it is also necessary to maintain this flatness during the heat treatment involved in coating the electrode with an electrocatalytically-active coating. Furthermore, great care must be exercised in assembling units in a bipolar electrolyser if damage to the separators is to be avoided.
U.S. Pat. No. 4,557,816 discloses that uniform concentration distribution of electrolyte across the electrodes in the bipolar electrolyser described therein may be improved by the provision of certain vertical ducts on the side of the electrode away from the separator for liquid downflow.
U.S. Pat. No. 4,643,818 discloses that the electrical resistance of the bipolar electrolyser described therein may be lowered and uniform current density obtained by the use of certain rigid multi-contact electrical conductive means between individual cells in a bipolar electrolyser.
U.S. Pat. No. 4,734,180 discloses a bipolar-type electrolyser comprising bipolar units which are arranged back to back with an explosion-bonded titanium-iron plate disposed therebetween and in which conductive ribs are welded to the electrode and to the pan-shaped body
U.S. Pat. No. 5,225,060 discloses that the formation of gas zones in the upper portions of anode and cathode structures of the bipolar electrolyser described therein may be prevented by the use of non-current-flowing space in the upper portion of the structures.
EP 0,704,556 discloses that the provision of gas-liquid separators at the top of the anode and cathode structures of the bipolar electrolyser described therein minimises pressure fluctuations, membrane deterioration and voltage variation.
WO 98/55670 discloses an element for a bipolar electrolyser the two sides of which are formed with flanges and opposed projections. The projections are preferably in form of truncated cones and are preferably arranged in the form of centred hexagons to improve lateral mixing of electrolyte. However, vertical mixing is achieved by downcomer ducts which decrease the active area of the electrode thus amplifying the current density variation across the electrode which is a particular problem when operating at high current density with a narrow or zero gap anode/membrane/cathode configuration with impurity-sensitive ion-exchange membranes.
EP 0 521 386 discloses a bipolar electrolyser comprising electrolytic cell units which comprise electrode partitions with mating recesses and projections with electrode meshes joined directly, or through a spring, to the projections. A stack of such upright electrolytic cell units are joined in series with an ion exchange membrane between adjacent electrolytic cell units to form an electrolytic cell assembly. However, whereas the recesses and projections ensure good lateral mixing of the electrolyte, vertical mixing is poor. Accordingly, to maintain concentration and temperature variation in the cell units within acceptable limits, energy-intensive pumped external recirculation is necessary with additional expensive pipework and tanks.
In electrolytic cells in which the anode-cathode gap is zero there is a tendency for pressure applied to the separator through the anode and cathode with which it is in contact to lead to deviations from uniformity, abrasion or even rupture, of the separator. This is particularly the case where the separator is an ion-exchange membrane where it is desirable to apply an even pressure to the membrane through the anode and cathode.
We have now devised an electrode structure for a bipolar electrolyser which allows very small or even zero anode/cathode gaps to be used in such electrolysers without damage to the separator, which minimises electrical resistance by using a short perpendicular current-carrying path length between electrodes and low resistance materials for almost the entire perpendicular current-carrying path length and which affords excellent current distribution throughout the electrode area. The electrode structure permits both horizontal and vertical flow of liquors therein aiding circulation and mixing thereof and has improved rigidity and strength which allows closer tolerance to be achieved in cell construction. The electrode structure is of simple construction and easy to fabricate.